The nuclear lamina is a protein meshwork lining the nucleoplasmic surface of the nuclear envelope (NE) that contains a polymeric core of nuclear lamins and associated transmembrane (TM) proteins. The lamina has a fundamental role in organizing chromatin structure and in controlling signaling and gene expression. Over 20 human genetic disorders are caused by mutations in lamina proteins, mostly in lamins A/C and in TM proteins associated with the lamina. These laminopathy diseases predominantly affect muscle, bone and adipose tissue, which are formed from mesenchymal progenitors. A systematic analysis of the NE/lamina proteome of cells of bone and adipose tissue has not yet been carried out. Osteoblasts and adipocytes differentiate from mesenchymal stem/stromal cells (MSCs). The choice between these alternative cell fates is specified by signaling pathways that induce distinctive transcription factor programs. Many of the transcription factors that promote one pathway antagonize the other, including the central regulators of osteoblast differentiation-Runx2 and Osterix, and the core factors for adipogenesis-PPAR? and C/EBP family members. Based on preliminary studies and the results of previous proteomics, a distinctive profile of nuclear membrane proteins arises in a cell type-specific manner during differentiation. This project will test the hypothesis that TM proteins of the NE that are differentially induced in osteoblast vs. adipocyte differentiation are integral parts of the signaling networks for generatio of each these lineages. Aim 1 will use proteomics analysis to systematically compare the profile of NE TM proteins of adipocytes and osteoblasts that emerges during differentiation. Aim 2 will use RNAi screens to investigate the role of lineage-selective NE proteins in adipocyte vs. osteoblast differentiation, and to identify different transcription factor pathways that are linkedto specific proteins. This work is expected to delineate how the changing composition of the nuclear lamina during differentiation of mesenchymal progenitors promotes the alternative osteoblast and adipocyte fates. This insight could enhance an understanding of laminopathies involving skeletal tissue. More generally, the work will be relevant to understanding skeletal maintenance in pathophysiological conditions and aging, since continued differentiation of mesenchymal progenitor cells into adipocytes, chondrocytes and osteoblasts is an ongoing process throughout adult life.